1. Field of the Invention
The present invention relates to an optical waveguide device. More particularly, the present invention relates to an optical waveguide device having a substrate comprising, as a principal component, lithium niobate, and an optical waveguide, exhibiting a reduced DC drift, and thus useful as an optical wave modulator. The optical waveguide device of the present invention is advantageous in that even under an application of a controlling DC voltage, the device can exhibit a stabilized operating property and is useful as a key element, for example, a high speed optical intensity modulator or a high speed optical switch, of an optical communication system, an optical information treatment system or an optical measurement system.
2. Description of Related Art
In conventional optical waveguide devices having a lithium niobate substrate, a generation of DC drift is a big problem to be solved. It is known that the mechanism of the generation of the DC drift can be explained by using an RC equivalent circuit model, as described in:
S. Yamada and Minakata, Jpn. J. Appl. Phys., Vol. 20 (1981), 733, PA1 C. M. Gee, G. D. Thurmond, H. Blauvetl and H. M. Yen, Appl. Phys. Lett., Vol. 47 (1985), 211, and PA1 R. A. Becker, Opt. Lett., Vol. 10 (1985), 417.
Namely, it is known that the amount and direction of the DC drift of an optical element greatly depend on the electric resistance (R) and the electric capacitance (C) of each of the lithium niobate substrate and a dielectric layer formed on the substrate of the optical element, especially on a relaxation time (.tau.) of the element which is defined as a product (RC) of the electric resistance (R) and the electric capacitance (C).
From the above-mentioned facts, it is understood that the DC drift of the optical element can be controlled by regulating the electric resistance (R), electric capacitances (C) and relaxation times (r) of the substrate and dielectric layer of the optical element.
As particular means for controlling the parameters of the optical element-forming materials, the following techniques have been reported.
According to H. Nagata, J. Ichikawa, M. Kobayashi, J. Hidaka, H. Honda, K. Kiuchi and T. Sugamata, Appl. Phys. Lett., Vol. 64 (1994), 1180, the amount of DC-drift can be restricted by reducing the content of hydrogen in the substrate. Particularly, since hydrogen is introduced into the lithium niobate substrate during a poling treatment of the substrate crystal, the hydrogen content in the substrate can be easily reduced by heat-treating the substrate in a dry atmosphere. The mechanism of the relationship between the hydrogen content of the substrate and the DC drift of the substrate is not fully clear. Nevertheless, it is assumed that since the electric resistance (R) of a low hydrogen content substrate is usually higher than that of a high hydrogen content substrate, this phenomenon probably contributes to the reduction in the DC drift of the low hydrogen content substrate.
Usually, the dielectric layer is formed from silicon dioxide. Also, it is known from, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-257,105 that the DC drift of the dielectric layer can be reduced by doping silicon dioxide with at least one member selected from the elements of the III to VIII groups, Ib group and IIb group of the Periodic Table. This technique appears to correspond to the control of the electric resistance (R), electric capacitance (C) and relaxation time (.tau.).
From another report, it is known that the DC drift of the lithium niobate substrate can be reduced by surface-modifying the substrate by an ion-injection method prior to the formation of the dielectric layer on the substrate.
In still another report, it is known from, for example, Minakata, Electronic Information Communication Association Theses, Vol. J77-C-1 (1994), 194, that the DC drift of a lithium niobate substrate can be reduced by etching and removing a surface portion of the substrate, in which the composition of the materials from which the substrate is formed, is modified during a step for forming an optical waveguide in the surface portion of the lithium niobate substrate.
As mentioned above, various methods have been attempted to reduce the DC-drift. However, these methods are not satisfactory for practical use. Also, the known methods were experimentally discovered and developed, and thus do not fully correspond in numerical results to the RC equivalent circuit model.
Although the known methods are practically contributory to reduce the DC drift, it is difficult to exactly evaluate the practical usage of the known methods.
However, from the viewpoint of industrial production of the optical device, it is most desirable that the DC-drift-reducing method is as simple as possible and can be easily controlled. For example, with the known method, in which one or more doping elements are forcedly introduced into a dielectric layer, it is difficult to quantitatively control the introduction of the elements with a high reproducibility.
In the production of an optical waveguide device, for example, a Mach Zehnder optical modulator, in a surface portion of a lithium niobate (LiNbO.sub.3) substrate, an optical waveguide having a pair of branch portions converged in a Y-shape into an input portion and output portion thereof is formed by a thermal diffusion method of titanium, a dielectric (buffer) layer is formed from a dielectric material on the substrate surface, and an electrode system composed of a plurality of electrodes is arranged on the dielectric layer. This production process is disclosed, for example, by Nishihara, Haruna, and Sumihara, "Optical Integrated Circuits", 1985, Ohmu-Sha. Optionally, for the purpose of evenly dispersing an electric charge generated on the surface of the optical device due to a pyroelectric effect of LiNbO.sub.3 when the environmental temperature, at which the optical device is operated, is changed, without allowing the charge to be concentrated under the electrodes, and of reducing a drift in the device properties (operating points of modulated phase), a thin layer having a lower electric resistance than that of the dielectric layer is arranged on or under the dielectric layer, as disclosed in K. Seino, T. Nakazawa, Y. Kubota, M. Doi, T. Yamane and H. Hakogi, `Proc. OFC` 92, San Jose, Feb. 8 to 11, 1992 (Optical Soc. An. Washington (1992), 332, Jumonji, and Nozawa "Electronic Information Communication Association Theses," C-1, J75-C (1992) 17, and Japanese Unexamined Patent Publication No. 5-66,428.
In the formation of the optical waveguide in the surface portion of the lithium niobate substrate, the thermal diffusion treatment with titanium is carried out at about 1000.degree. C. To reduce or prevent an undesirable out-diffusion of lithium from the substrate at the high temperature to the outside of the substrate, the thermal diffusion treatment is usually carried out in a wetted gas atmosphere provided by flowing the gas through a water bath while bubbling, as disclosed in the abovementioned "Optical Integrated Circuits". In most all of the above-mentioned treatments, the wetted thermal diffusion atmosphere is provided by wetting a pure oxygen gas, an argon-oxygen-mixed gas or air.
The dielectric (buffer) layer, which is an important constituent of the optical element, is formed on the optical waveguide-provided substrate from a dielectric inorganic oxide, for example, SiO.sub.2 or Al.sub.2 O.sub.3, by a vacuum deposition method, ion-assist vacuum deposition method, sputtering method or chemical vapor deposition method, which is a common method of forming a thin film.
The inorganic oxide, for example, SiO.sub.2 or Al.sub.2 O.sub.3 has a high electrically insulating property and exhibits a low dielectric constant (refractive index). Therefore, optical signals being transmitted through the optical waveguides and high frequency electric signals being transmitted through the electrodes can be matched together and retained at a high efficiency. Also, since both the substrate-forming material and the dielectric layer-forming material are inorganic oxides, the interfacial chemical bonding force between the substrate and the dielectric layer is strong thus the dielectric layer can be bonded to the substrate surface with a high stability.
It is known from Japanese Unexamined Patent Publication (Kokai) No. 58-181,318, that after the dielectric layer is formed by the above-mentioned method, for example, a vacuum deposition method or sputtering method, the resultant dielectric layer is heat-treated in an oxidative atmosphere at a temperature of about 600.degree. C., to eliminate an oxygen defects in the dielectric layer. In this thermal treatment, usually a wetted oxygen gas prepared, for example, by bubbling the oxygen gas through a water bath, is employed to reduce the out-diffusion of lithium and to promote the oxidation reaction of the dielectric layer.
The above-mentioned conventional methods including the thermal diffusion step for forming the optical waveguide in a wetted gas atmosphere and the heat treatment step for the dielectric layer in a wetted gas atmosphere, exhibit an unsatisfactory DC drift phenomenon which is a big problem to be solved for the Mach Zehnder optical wave modulator having a lithium niobate substrate.
The term "DC drift phenomenon" is referred to as a phenomenon of changing an adjusted, modulated optical phase of an optical device output and of getting out of the adjusted phase, with a lapse of time due to a DC voltage applied to the device. The DC drift phenomenon is not fully clear and is still under discussion: In one prevailing opinion, it is assumed that the DC drift phenomenon closely relates to electric properties (for example, resistivity and dielectric constant) of the substrate and the dielectric (buffer) layer, and thus it is difficult to completely prevent the DC drift phenomenon of the optical device. (Jumonji and Nozawa, Electronic Information Communication Theses, C-1, J75-C-1 (1972), 17, and Japanese Unexamined Patent Publication (Kokai) Nos. 5-66,428 and 4-346,310)